A Weighing Pipet to Hold Liquid Samples for Dry CombustionA. Russell Jones Citation: Review of Scientific Instruments 24, 230 (1953); doi: 10.1063/1.1770674 View online: http://dx.doi.org/10.1063/1.1770674 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/24/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in When Liquids Stay Dry Phys. Today 51, 38 (1998); 10.1063/1.882133 Application of hydrostatic weighing to density determination of tiny porous samples Rev. Sci. Instrum. 66, 2578 (1995); 10.1063/1.1145591 Using a sample and hold circuit as a boxcar integrator/ultrasonic recording unit Am. J. Phys. 47, 1012 (1979); 10.1119/1.11668 Hydraulically Actuated Combustion Gas Sampling Valve Rev. Sci. Instrum. 39, 1820 (1968); 10.1063/1.1683248 Combustion Gas Sampling Valve Rev. Sci. Instrum. 36, 1028 (1965); 10.1063/1.1719751

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230 LABORATORY AND SHOP NOTES

FIG. 1.

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photometer. It is patterned after the liquid microcell but is modified to hold silver chloride plates l by ! inch with a thickness of 1 millimeter. The combined thickness of plastic holder and silver chloride plate places the sample approximately in the conjugate focal plane of the entrance slit. The accurate location of the slit focus within that plane is important in order to use the smallest possible amount of sample. This can be accomplished by cutting a mask slightly larger than the largest slit to be used, mounting the mask in the holder, and then moving the adapter until the transmission is equal to its value without the mask. The corre­sponding area may be etched on the silver chloride with a sharp instrument as a guide in preparing the sample.

Using this sample holder satisfactory spectra have been obtained rOlltinely with samples of 0.4 milligram.

A Weighing Pipet to Hold Liquid Samples for Dry Combustion *

A. RUSSELL JONES

Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee (Received July 26, 1952)

I NVESTIGATIONS in progress at this laboratory required that a large number of dry combustions of liquid samples (in

pa,rticular, water solutions) be performed. It has been found possible to burn liquid aliquots of any desired

size by the use of a reusable weighing pipet constructed of quartz (Fig. 1). The pipet is about 5 cm long. A micropipet syringe, roughly calibrated by scratches on the cylinder and piston, is used to suck the desired amount of liquid into the pipet. The capillary is emptied by continued withdrawal of the piston. The wet end is then wiped and briefly flamed. Oscillation of the piston clears the capillary. With larger samples the liquid quickly runs to the

FIG. 1. Weighing pipet.

ends of the cylinder (dotted lines) and so tightly surrounds the seal that it is difficult to dislodge.

The pipet is weighed with (and placed in the furnace in) a small platinum wire support which serves both to hold it in place during weighings and to keep it from contact with copper oxide dust on the bottom of the combustion tube. If contact is not prevented, the pipet gradually becomes opaque.

Samples containing as little as 0.01 mg of carbon in the form of a C14-labeled compound dissolved in 0.2 ml of water are quantita­tively burned in an ordinary semimicro combustion tube.

• This document is based on work performed under Contract Number W-7405-eng. 26 for the U. S. Atomic Energy Project at the Oak Ridge National Laboratory.

A Controlled Gas Leak JAMES MORRISON

Bell Telephone Laboratories, Inc., Murray Hill, New Jersey (Received September 5, 1952)

I N many vacuum studies, it is desirable to admit gas into a system at controlled rates covering a wide range. Described

in this note is a leak system whereby a steady pressure in the range from 5.10-7 to 5.10-3 millimeters can be maintained in a vacuum system which is being exhausted continuously at a rate of approxi­mately 2 to 3 liters per second.

The leak system employs porolls porcelain rods similar in material to those described by Hagstrum and Weinhart.1 The rate of flow of gas through the rod is determined by the pressure of gas. Area is controlled by the displacement of surrounding mercury, using a magnetically operated plunger. In the two leaks of Fig. 1, two ceramic rods of different diameters are shown. These

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LABORATORY AND SHOP NOTES 231

GAS BOTTLE

LEAK NO. I

FIG. 2.

LEAK NO.2 TO TRAP AND

HEADER

rods were made in these different diameters in order to obtain a wide range of leak rates. As a further refinement the small diam­eter rod in the second leak was ground to a tapered point.

In the. system, s~own in Fig. 2, high capacity and wide range are obtamed by usmg two leaks and two reservoirs, all in series. The first leak controls the pressure in a reservoir between the two. This reservoir is set at a high pressure if a high over-all leak rate ~s de~ir~d and a .low pressur.e if a low rate is desired. The pressure m thiS mterme~hate reservOir may be measured by' a Pirani gauge.

The system IS evacuated to a low pressure by pumping through the stopcocks which by,pass the ceramic leaks. These are precision ground stopcocks lubricated by a low vapor pressure grease. When the system is evacuated, stopcock A is closed and the break-off tip on the gas reservoir is broken. Stopcocks Band C are closed, and the plunger in the first leak is raised exposing the rod. The pressure is adjusted in the intermediate reservoir and measured by the Pirani gauge. Then leak number one is cut off, and leak number two is adjusted to the desired rate of flow.

The use and positions of the stopcocks result in flexibility in the leak rate as well as making it possible to evacuate all sections of the system. Either one or both of the leaks can be by'passed if nece~sary; this is possibly due to depletion of the gas supply with contmued use.

The advantages of this leak system are (1) a controllable range of 10000 to 1 in leak rate; (2) no long mercury columns are ne~essary to compensate for pressure differentials; (3) gas reser­VOirs can be used when filled to atmospheric pressures; (4) the leak rate can be held constant for many hours.

We wish to thank Mr. W. H. Aug of the Laboratories for the glasswork on the leak details.

1 H. D. Hagstrum and H. W. Weinhart. Rev. Sci. lnstr. 21. 394 (1950).

Magnetostrictive Sonic Delay Line H. EpSTEIN AND O. STRAM

Burroughs Addin,g Machine Company. Research Division. Phs/adelphia Z3. Pennsylvania

(Received June 3, 1952)

I N view of the present activity in the field of magnetostrictive sonic delay lines, our experience in the development of such a

device may be of interest. A sonic delay line is a device in which electrical pulses are con­

verted by means of a transducer into mechanical vibrations which then traverse a body of material called the delay element at the velocity of sound in that particular material until the vibrations reach another transducer at which point the mechanical vibra­tions are reconverted into electrical pulses. The slow travel of mechanical disturbances in certain solid materials as compared with the velocity of propagation of electrical signals makes possible a relatively long delay in a device of small size. The magnetostric-

tive sonic delay line is composed of a delay element (in such forms as rod, ribbon, tubing, or the like) made of nickel or other materials exhibiting magnetostrictive properties with transducer coils along the element. The conversion from electrical to mechanical pulses at the transmitter and from mechanical to electrical pulses at the receiver is accomplished respectively by the use of the Joule and Villari magnetostrictive effects.

The purpose of the investigation is primarily to develop a storage system for a low speed digital computer as well as the app1icatio~ of.the device as a continuously variable delay line, as a pulse dlstnbutor, for serial to parallel conversion and as multiplying devices. '

Present opinion holds that ribbon shaped delay elements have inherently better frequency response than do tubes or other elements of circular cross section. Our experience shows, however, that by choosing tubing of proper dimensions adequate high frequency response can be obtained without the necessity of slotting the tubing along its length, since, in any case, the fre­quency response is ultimately limited by eddy current losses in the transducers. In fact, the frequency response of a tubular element may in some cases be superior to that of a ribbon whose width is the same as the circumference of the tubing and which has undergone the same heat treatment.

The use of tubing instead of ribbon or small diameter wire provides several advantages. Tubing is inherently a more rugged structural form and is self-supporting in moderate lengths. The fact that a self-supporting delay element does not have to be held un~er tension allows the easy construction of long, continuously va:lable delay lines and in general simplifies packaging problems. It IS also noted that magnetic coupling between circular transducer coils and a delay element of circular cross section is better inas­much as the air gap can be made smaller, resulting in somewhat smaller insertion loss as well as better resolution resulting from less fringing of the flux.

To date, the results of our investigation have been a laboratory test model and prototype models of a recirculating memory unit with a storage capacity of 125 bits at a 450 kc repetition rate and a four-channel pulse distributor.

The laboratory test model has as a delay element a three-foot piece of thin wall 0.045 in. o.d. nickel tubing. The transducer coils are wound on fiber spools with 250 turns of No. 40 wire and fitted with Ferroxcube IV ferrite cups which totally enclose the coil with the exception of the center hole. The coils are encapsulated in resin. The length of the ferrite cups is adjusted to correspond with one­half the wavelength of <J. 6OO-kc sinusoidal vibration in the nickel tube. A permanent magnet is mounted at the receiver coil to supply the biasing flux necessary for the Villari effect to be ex­~ibited. (It should be noted that if an external permanent magnet IS used there must be enough of an air gap between the ferrite return and the nickel tube at the center hole of the coil to prevent the "shorting" of the biasing flux by the ferrite.)

The transducer coils are fastened by suitable mountings to a piece of aluminum channel. The ends of the nickel tubing are lightly held by clamps at the ends of the channel to prevent movement of the tube and to insure good grounding. Adequate echo suppression was simply obtained by coating portions of the nickel tubing near the ends with a sticky beeswax mixture.

The transmitting coil is driven by a single 6AG7 the grid of which is normally at cutoff, but which is driven positive by a 30-volt pulse from a pulse control unit. This excites the coil with a current pulse of approximately 250 milliamperes. The low signal level at the receiver plus the high impedance the coil offers at the operating frequencies makes it advisable to use a preamplifier to avoid noise pick-up in the leads.

The following characteristics were measured using a line of approximately 100-microseconds delay, although an experimental model of a continuously variable delay line of somewhat more than 800 microseconds has been constructed.

(1) A delay of about 5.27 microseconds per inch of nickel tubing makes possible a continuously adjustable delay over the range

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